In semiconductor lithography (or photolithography), the fabrication of an integrated circuit (IC) includes performing a variety of physical and chemical processes on a semiconductor (for example, silicon) substrate (which is also referred to as a wafer). A photolithography exposure apparatus or scanner is a machine that applies a desired pattern onto a target portion of the substrate. The wafer is irradiated by a light beam that extends along an axial direction, and the wafer is fixed to a stage so that the wafer generally extends along a lateral plane that is substantially orthogonal to the axial direction. The light beam has a wavelength in the deep ultraviolet (DUV) range, for example, from about 10 nanometers (nm) to about 400 nm. The light beam travels along the axial direction (which that is orthogonal to the lateral plane along which the wafer extends).
A spectral analysis module is used to measure spectral features of the light beam, and such measured spectral features are used to control aspects of the light beam. By controlling the light beam, various lithography properties can be controlled, For example, a minimum feature size or critical dimension (CD) at the wafer can be controlled, or pattern properties such as overlay, surface roughness, and proximity correction can be controlled.